An entrance stone (5)
I believe the answer is:
agate
'stone' is the definition.
(agate is a type of stone)
'an entrance' is the wordplay.
'an' becomes 'a'.
'entrance' becomes 'gate' (type of entrance).
'a'+'gate'='AGATE'
(Other definitions for agate that I've seen before include "Quartz with bands of colour, used in jewellery" , "Stone with coloured bands" , "Ornamental form of quartz" , "rock with bands" , "Semi-precious quartz" .)